[brianogilvie]

Not as far as I know. OmniFocus doesn't use dates for accounting; it uses them for determining what you can do now. From that point of view, there's no point in setting a start date to today, or in the past, because the action is already available without the start date. It only makes sense to set a start date in the future, which prevents the action (or project) from becoming immediately available (as long as other conditions for being available are also met, e.g. being in a context that is not on hold, and being the first action if it is in a sequential project).